Chromosomal Abnormalities and Bipolar Affective Disorder: Velo-Cardio-Facial Syndrome

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Medscape Psychiatry & Mental Health eJournal. 1997;2(4) 

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Glossary of Genetic Terms

Genome The entire set of chromosomes
Base, megabase A base is a single nucleotide in DNA (A, C, T, or G). A megabase is 1 million bases
Chromosome maps (eg, 22q11) Human chromosomes have long arms (referred to as p) and short arms (q), and the arms are further subdivided into numbered segments. Therefore, chromosome 22q11 would be the short arm of chromosome 22, segment 11.
Monozygotic twins Twins from the same egg ("identical")
Dizygotic twins Twins from separate eggs ("fraternal")
Alleles Different forms of a gene. For instance, different alleles for a gene for an enzyme may code for enzymes with different enzymatic rates.
Genetic locus Location of a gene on a chromosome
Linkage analysis, segregation analysis Determination of whether 2 genes are linked, or located on the same chromosome. Genes on different chromosomes show no linkage. For genes on the same chromosome, the amount of linkage indicates how close together the genes are.
Pedigree A multigenerational family
Types of inheritance
Mendelian One gene determines 1 trait
Autosomal dominant, autosomal recessive Not all alleles have equal influence. In a heterozygote, 1 of 2 different alleles will overrule the other. The gene that has the greater influence is called the autosomal dominant. The gene that is not expressed, in this circumstance, is called the autosomal recessive. An autosomal dominant trait will ordinarily be expressed if the dominant gene is present in either member of the chromosome pair. In autosomal recessive inheritance, the recessive characteristic appears only when the individual is a homozygote for the recessive gene.
X-linked dominant Inheritance is rare, although early linkage studies found a possible association with bipolar disorder. The mode of transmission does not permit a father to transmit an x-linked dominant trait to his son, but all of his daughters would be affected.
X-linked recessive Traits follow a distinct pattern of transmission that leads to the skipping of generations before they recur. The most common example is hemophilia.
Penetrance The probability that a person will manifest the disease if he or she has the gene for the disease.
Genetic heterogeneity The presence of more than 1 allele in a population.
Polymorphism and polymorphic DNA markers The presence of more than 1 type of marker at the same chromosomal site in a population (similar to genetic heterogeneity).
Sib-pair analysis The sib-pair method is one approach to linkage analysis. It does not require any assumptions about the genetic mechanisms involved in the disease, but is statistically less powerful than family linkage analysis.
Chromosomal (cytogenetic) abnormalities Alterations of chromosomes that can be seen under a microscope
Aneuploidy Loss of 1 of a pair of chromosomes (opposite of polyploidy, which is more than 2 copies of a chromosome, as in trisomy 21, Down's syndrome)
Translocation Shift of a piece of a chromosome to another chromosomal location
Deletion Loss of a segment of a chromosome
Codon Every 3 continuous nucleotide bases represented on the DNA and mRNA, called a codon, encode for a specific amino acid residue. Starting at a specific "start" signal, mRNA is translated into a protein, the amino acid sequence of which is predetermined by the order of codons.
Pleiotropy Different manifestations of the same genetic defect
Homozygous Presence of 2 copies of the same allele
Hemizygous Presence of only 1 copy of a gene (because of deletion, or in a male, because the gene is on the X chromosome)
Heterozygous Presence of 2 different alleles of the same gene
Genotype The genetic make-up of an organism
Phenotype The physical expression of the genotype


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